Toner binders for use in electrophotography adopting a heat fixing system, which is a system of image fixation commonly applied in copiers, printers, and the like, are required to be capable of fixing a toner even at a low fixing temperature (low-temperature fixability), and to have heat-resistant storage stability and storage stability under a high-temperature and high-humidity environment (moisture heat-resistant storage stability). Development of a toner binder capable of attaining a combination of low-temperature fixability with heat-resistant storage stability and moisture heat-resistant storage stability has been demanded because an attempt to improve low-temperature fixability will lead to deterioration in heat-resistant storage stability and moisture heat-resistant storage stability, whereas an attempt to improve heat-resistant storage stability and moisture heat-resistant storage stability will lead to deterioration in low-temperature fixability.
For example, a toner binder comprising an amorphous polyester having a furan ring for attaining a combination of low-temperature fixability and heat-resistant storage stability has been proposed (Patent Document 1). Moreover, methods using an aromatic monocarboxylic acid (Patent Documents 2, 3), a toner binder comprising one or more species selected from aliphatic monools and aliphatic monocarboxylic acids (Patent Document 4), toner binders prepared by reacting a polyester resin with an isocyanate compound (Patent Documents 5, 6), a polyester in which 30% or more of hydroxy groups or acid groups are capped with a masking agent (Patent Document 7), etc. have been proposed.
Furthermore, as a general matter, an ingredient contained in a toner might attach to a member constituting a copier or a printer, for example, a developer tank which stores a developer therein, and this might cause image degradation.